Energy-dispersive X-ray spectroscopy (EDS, EDX, or XEDS), sometimes called energy dispersive X-ray analysis (EDXA) or energy dispersive X-ray microanalysis (EDXMA), is an analytical technique used for the elemental analysis or chemical characterization of a sample. It relies on an interaction of some source of X-ray excitation and a sample. Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure allowing unique set of peaks on its X-ray spectrum.
An EDS system generally includes an excitation source (e.g., electron beam or x-ray beam), an X-ray detector, a pulse processor and an analyzer. An X-ray detector is used to convert the collected X-ray energy into voltage signals which are in turn sent to a pulse processor. The pulse processor measures the signals and passes them onto an analyzer for data display and analysis. The most common detector is Si(Li) detector cooled to cryogenic temperatures with liquid nitrogen. Silicon drift detectors (SDD) with Peltier cooling systems are also used.
Specifically, to stimulate the emission of characteristic X-rays from a sample, a high-energy beam of charged particles such as electrons or protons (e.g. in particle-induced X-ray emission or proton-induced X-ray (PIXE)), or a beam of X-rays, is focused into the sample being studied. At rest, an atom within the sample contains ground state (or unexcited) electrons in discrete energy levels or electron shells bound to the nucleus. The incident beam may excite an electron in an inner shell, ejecting it from the shell while creating an electron hole where the electron was. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lower energy shell may be released in the form of an X-ray. The number and energy of the X-rays emitted from a specimen can be measured by an energy-dispersive spectrometer. As the energy of the X-rays is characteristic of the difference in energy between the two shells, and of the atomic structure of the element from which they were emitted, this allows the elemental composition of the specimen to be measured.
Scanning electron microscopes (SEM) systems often have a magnetic immersion lens at the front of an electron optical column proximate the sample. The magnetic immersion lens typically has two pole pieces with rotational symmetry about a central axis of the electron optical column A magnetic field is produced in the pole pieces by one or more pairs of current carrying coils. There is a gap between the two pole pieces, which form a magnetic circuit. Fringing fields in the region near the gap focus or deflect electrons from the optical column. X-rays emitted from the target can pass through the gap to the X-ray detector. In SEM systems, it is desirable to increase access to X-rays emitted from the sample.
Previous methods for increasing access to the sample include moving sample further away from the lens, or increasing the gap between the lens pole pieces. Each of these ways of increasing access to the sample has disadvantages. Moving the sample away increases the electron spot size and thus decreases imaging resolution. Increasing the gap between magnetic pole pieces increases the magnetic reluctance of the circuit, thus requiring more current to achieve the same magnetic field. This in turn increases the heat dissipation within the lens which can have further deleterious effects on system performance.
It is within this context that aspects of the present disclosure arise.